FLUCTUATIONS IN A PARTRIDGE POPULATION 129 



lected that have died as a result of 'chilling', but only a small proportion of 

 the chicks found are sufficiently fresh to allow the cause of death to be 

 ascertained. Lack of evidence of death from disease does not necessarily mean 

 that this has not occurred, although the very few known cases of deaths of 

 young birds from Coccidiosis or 'Blackhead' suggest that their effect on 

 survival rates is neghgible. The gapeworm [Syngamus trachea) has, however, 

 increased very markedly over the period 1949-59, and its incidence, although 

 unknown in chicks, has been fairly well charted in adults and juveniles. There 

 is some evidence that the heavy rate of chick loss is not necessarily affected 

 by this disease, but its incidence among partridge chicks is unknown. There 

 was no evidence of this disease in 1949, and a gradual spread from a single 

 focus of infection appears to have begun in 195 1. Four years later the degree 

 of infection in juveniles shot in October was 45 per cent, and although the 

 area originally infected showed the highest incidence of this disease, the 

 survival rate in this area was at least as high as on the areas least affected. By 

 the end of 1958 all parts of the 4,000-acre (1,600 hectare) area showed some 

 degree of infection, while the highest incidence was still found in the area 

 of the original outbreak. In 1959, probably due to the unusually dry summer, 

 the incidence of Syngamus decreased, although it was still present over the 

 whole area. 



A comparison of the breeding pair density in March with the subsequent 

 young/old ratios in August on the summer-gain figures obtained from the 

 September censuses, shows that there appears to be a well-marked inverse 

 ratio. Low breeding densities have been followed by high recruitment rates 

 (1949) and high densities by low recruitment rates (1954). If breeding densities 

 and young/old ratios were the only available . data, the correlation might 

 appear to be significant. But when we consider the sequence of weather 

 during the breeding season, as well as the changes in crop distribution, the 

 partly fortuitous nature of the correlation becomes apparent. It is perhaps 

 unnecessary to repeat that a crop distribution that is favourable to a high 

 breeding density, often prevents the occurrence of a high production rate 

 due to the extent of the grass-leys. Variation in the average number of chicks 

 hatched per pair (depending on clutch-size, hatchability and nesting success) 

 is relatively small, and shows no correlation with the breeding density. 

 Although nesting success appears to be unaffected by breeding density, chick 

 survival may be affected, since a larger proportion of pairs may nest in 

 unfavourable situations where the chances of rearing a brood of chicks 

 successfully are very small. However, it has proved impossible to measure 

 this tendency on any quantitative basis. 



No satisfactory method of measuring the amount of chick loss due to 

 predation has been discovered, but it is unlikely that large annual variations 



